The present invention relates to a variable-capacity gas compressor for use in, for example, an automotive air conditioner, and more particularly to a gas compressor which is capable of varying the cooling power during operation.
In general, a gas compressor used in an air conditioner of a passenger car is mounted besides the engine and is adapted to be driven by a crankshaft pulley of the engine through a V-belt. The compressor is provided with an electromagnetic clutch which selectively transmits the driving power from the engine to the compressor.
Therefore, the power of the gas compressor is increased in proportion to the speed of the engine. This means that, when the automobile is running at a high speed, the gas compressor is also driven at a high speed with the result that the air in the compartment is cooled excessively. In addition, power consumption also is increased correspondingly. (Cooling power characteristics of the gas compressors of this type are shown by broken-line curves in FIG. 7.)
In order to overcome these problems, the same applicant has filed copending U.S. application Ser. Nos. 902,419, and 902,421 which disclose gas compressors of variable-capacity type, capable of varying the volume of the compression chambers in accordance with the speed of the gas compressor.
A typical example of such variable-capacity gas compressors will be explained hereinunder. In this variable-capacity gas compressor, a control plate is provided on the inner surface of the front side block of the compressor, and a recess (suction port) which communicates with a communication port in the front side block and the cylinder chamber, is formed in the control plate. The control plate is adapted to be rotated through a predetermined angle by a driving means, so as to vary the volume of the gas which is sucked through the communication port in the front side block.
However, the compressor in which the capacity is varied by the control plate encounters the following problem. Namely, it takes a much longer time for this type of gas compressor to cool the air to a set temperature as compared with conventional gas compressors (compression chamber volume is constant) the characteristics of which are shown by the broken-line curve in FIG. 7. Thus, the known variable-capacity type gas compressor of the type described could not comply with the demand for quick cool down of air, which demand is particularly acute in daytime of the summer season. (line curve in FIG. 7).
The long time required for the variable-capacity compressor to cool the air down to the set temperature is attributable to the fact that the suction pressure in the suction chamber is maintained constant.
FIG. 9 is a graph which shows the values of cooling efficiency achieved by a variable-capacity type gas compressor which operates with the suction pressure in the suction chamber maintained at constant levels of 0.5, 1.0, 1.5 and 2.0 kg/cm.sup.2.
As will be seen from the graph shown in FIG. 9, there is a tendency that the lower the suction pressure, the shorter the time required for cooling the air down to the set temperature. It is also understood that the set temperature can be lowered if the suction pressure can be reduced.
In view of the above-explained relationship between the suction pressure and the cooling efficiency, the present inventors have found that a highly practical variable-capacity gas compressor can be obtained by designing such that, in the beginning period of operation, the compressor operates with reduced suction pressure so as to provide a high cooling efficiency, whereas, after elapse of a predetermined time, the compressor operates with raised suction pressure, so as to prevent any excessive cooling of air, and have found that an ideal variable-capacity gas compressor can be obtained by designing such that, in the beginning period of operation before the set temperature is reached, the compressor operates with a fixed capacity as in the case of the conventional gas compressor, whereas, after the set temperature has been reached, the compressor operates with the capacity-varying function.